1. Technical Field
The present invention generally relates to planar optical devices and, more particularly, to a planar optical device having reduced warpage and reduced polarization sensitivity.
2. Discussion
Planar optical devices generally include a substrate having thin glass layers deposited thereon. A typical device includes a silicon substrate and three glass layers. The three layers are generally known as the underclad layer which is deposited on the substrate, the core layer which is deposited on the underclad layer and the overclad layer which is deposited on the core layer.
For most applications, the polarization sensitivity of planar optical devices needs to be as low as possible. Polarization sensitivity of planar devices formed on silicon substrates is largely dependent on stress, including the warpage of the finished device. Polarization sensitivity is the wavelength shift, measured in nm, caused by the stress induced Birefrigence or TE-TM shift. This stress arises because of the huge mismatch in the coefficient of thermal expansion between the substrate and the glass layers.
One method that has been suggested for reducing warpage and therefore polarization sensitivity is to use glasses having high coefficients of thermal expansion for the overclad layer. For example, an overclad layer having a coefficient of thermal expansion approximately equal to 37xc3x9710xe2x88x927/C would be desirable on silicon substrates. Unfortunately, it is also desirable for the overclad layer to have an effective index of refraction which is approximately equal to the effective index of refraction of the underclad layer (1.457 for silicon, for example).
To date, no commercially available planar devices are known that have all glass overclad layers with such high coefficients of thermal expansion (e.g., 37xc3x9710xe2x88x927 /C) and an index of refraction which matches the index of refraction of the underclad layer. Further, no glass is known which is suitable for use as an overclad layer that has coefficient of thermal expansion greater than 27xc3x9710xe2x88x927/C that also has an effective index of refraction approximately equal to that of the underclad layer.
While optical devices incorporating an overclad layer having a coefficient of thermal expansion of 27xc3x9710xe2x88x927/C have yielded polarization sensitivities of 0.1 nm, this level of polarization sensitivity is unacceptable for future optical devices. This is particularly true for wavelength division multiplexers or other devices having a channel spacing less than 100 GHz.
In view of the foregoing, it would be desirable to provide a novel approach to achieving reduced stress, including warpage in planar optical devices to yield even lower polarization sensitivities.
The above and other objects are provided by an optical device including a substrate having a first coefficient of thermal expansion. An underclad layer with a first index of refraction is deposited on the substrate. A core layer is deposited on the underclad layer. The core layer has a second index of refraction which is greater than the first index of refraction. A first overclad layer is deposited on the core layer. The first overclad layer has a third index of refraction which is approximately equal to the first index of refraction. A second overclad layer is deposited on the first overclad layer. The second overclad layer has a second coefficient of thermal expansion which is greater than the first coefficient of thermal expansion.